In a conventional engine, the accessory box fitted with fuel pumps, bearing lubricant pumps, hydraulic pumps for controlling various members, electricity generators, and the starter, is located outside the engine and receives power taken from the engine by means of a radial shaft and angle takeoffs.
Over the years, the increases in the compression ratios and in the inlet temperatures to the turbine, and also the improvements in materials and efficiency, have led to a constant reduction in the size of engines so as to obtain an ever increasing ratio of weight/thrust, and this applies both to civilian applications and to military applications.
The power takeoff system and the accessory box have had difficulty in following this progress correspondingly, and they thus represent a large fraction of the volume and the weight of an engine, in particular of a low-thrust engine which is therefore small in size, particularly when the accessory box, generally placed on the engine casing, supports an air starter and an electricity generator that are separate.
The use of small engines, that are ever simpler and less expensive, for the purpose of propelling training airplanes, observation or attack drones, and cruise missiles, requires engine manufacturers also to make such engines more furtive. This can be attempted by greatly reducing their frontal surface area, which also achieves a significant saving in drag, making it possible to increase the flying time or the range of aircraft or of remotely controlled vehicles fitted with such engines. In order to reduce both weight and the frontal surface area of engines, it thus appears to be desirable to envisage integrating an electrical generator-starter in the engine and to eliminate the use of mechanical connections so that the interface between the engine and the accessories relies on electrical transmission.
With wide-bodied aircraft, having electrical or electrohydraulic flight controls in ever greater numbers, and also with radar, advanced warning, and electronic surveillance airplanes, electrical power requirements are large. The engines of such airplanes are fitted with booster or auxiliary generators, thus increasing the size of the auxiliary boxes, and also their weight in order to be able to carry the generators. In an engine having a large bypass ratio, it is therefore advantageous to integrate an auxiliary generator in addition to the generator-starter in the engine in order to reduce the size and the weight of the angle takeoff, or indeed in order to eliminate them, thereby obtaining a thinner cowl, by housing some of the electrically-driven accessories in the pylon.
In the prior art, integrating a generator inside the engine leads to using a cooling oil circuit specific to the generator in order to cool its coils, or indeed its magnets.
That increases the number of pieces of equipment (oil pipes and pumps) inside the engine, and also complicates access to the generator for maintenance purposes.